April 4, 2011 -- Girls who play high-impact sports like basketball, running, and cheerleading may be putting their bones at risk of developing stress fractures.

A new study shows adolescent girls who participate in high-impact sports are more likely to develop stress fractures than teenage girls who engage in moderate or non-impact sports like swimming, biking, dance, or baseball. In fact, each hour of high-impact sport activity increased the risk of stress fracture by 8%.

Stress fractures are tiny breaks in the bone, usually caused by repetitive stress. They can be quite painful, but in many cases heal themselves after a few months of rest.

Exercise promotes bone strength, but too much high-impact activity may take its toll on young, developing bones.

"Weight-bearing activity stimulates bone remodeling and thus increases bone mass density, but very high levels of activity may be detrimental to bone health and increase the risk of stress fracture," study researcher Allison Field, ScD, of Brigham and Women's Hospital in Boston, and colleagues write in the Archives of Pediatrics & Adolescent Medicine. "Although stress fractures are relatively uncommon, they affect as many as 20% of young female athletes and military recruits."

A Delicate Balance for Healthy Bones

In the study, researchers analyzed information from 6,831 girls between the ages of 9 and 15 who were daughters of women enrolled in the Nurses' Health Study II between 1996 and 2004.

During seven years of follow-up, 3.9% of the girls developed a stress fracture.

The results showed that girls who participated in eight or more hours of physical activity per week were twice as likely to develop a stress fracture as girls who were engaged in less than four hours of exercise each week.

But when researchers looked specifically at the type of physical activity, only three types of high-impact sports were independently associated with an increased risk of stress fracture: basketball, running, and gymnastics/cheerleading. Non-impact activities, including swimming, biking, skating, and skateboarding, and medium-impact activities like baseball, dance/aerobics, hockey, and karate did not increase the risk of stress fracture.

"Our study observed that high-impact activities, specifically basketball, running, and gymnastics/cheerleading, significantly increase risk for stress fracture among adolescent girls," write the researchers. "Thus, there is a need to establish training programs that are rigorous and competitive but include varied training in lower-impact activities to decrease the cumulative amount of impact in order to reduce the risk of stress fracture."

The study also showed that girls with a family history of osteoporosis or low bone mass density were twice as likely to develop stress fractures.

Finally, an older age at the start of a girl's menstrual period was also associated with a higher risk of stress fracture. Each one-year delay in the start of menstruation was associated with an approximate 30% increase in the risk of stress fracture.